Project 2: Targeting Immunosuppression Pathways to Enhance Brain Tumor Immunotherapy. John H. Sampson, M.D., Ph.D., M.H.Sc, Project Leader Glioblastoma multiforme (GBM) is the most common of primary malignant brain tumors and despite incapacitating conventional therapy, remains universally fatal. Immunotherapy is an attractive therapeutic alternative, but is limited by the lack of frequent and homogeneously-expressed tumor-specific antigens and a profoundly immunosuppressive host environment. This proposal addresses both of these limitations. The nearly universal presence and homogeneous expression of CMV antigens in GBM, but not normal brain, has now been well-established and provides an unparalleled opportunity to subvert these highly immunogenic viral proteins as tumor-specific targets. Despite the potential immunogenicity of CMV antigens, endogenous immune responses may be limited, at least in part, by an excessive number of immunosuppressive T-cells (TRegs). TRegs are a phenotypically distinct CD4+ CD25 + Foxp3+ population that normally prevent autoimmunity and are uniquely dependent on the high affinity interleukin (lL)-2 receptor (IL-2Ra/CD25) for their function and survival. Monoclonal antibodies (MAbs) that block lL-2Ra have been shown to abrogate Tpeg function in animal models but can also inhibit effective anti-tumor immune responses in mice and humans. Our preliminary data demonstrates that treatment with lL-2Ra blocking MAbs in mice undergoing homeostatic proliferation in response to transient lymphodepletion, as might be seen after therapeutic cycles of chemotherapy, still eliminates Tpegs, but in stark contrast, no longer impairs effector Tcell immune responses. Instead, vaccine-induced immune responses are strongly accentuated. This may be due to homeostatic surges in y receptor cytokines (IL-7 and IL-15) that share receptors with IL-2 and may bypass the need and effects of IL-2 signaling in activated T-cells in this context Preliminary clinical studies show that patients vaccinated with CMV pp65 mRNA transfected dendritic cells and then treated with a commercially-available MAb specific for the human IL-2Ra after therapeutic TMZ chemotherapy, have reduced Tpeg levels, increased pp65-specific CDS* T-cell responses, and progression-free survival interval of 27.4 months. In this proposal, we will test the HYPOTHESIS that during hematopoietic recovery from treatment-induced lymphopenia TRegs can be selectively inhibited by anti-lL2Ra MAbs to enhance anti-tumor immunity without the induction of limiting autoimmunity.